案例环境如下：
操作系统 ： centos7
数据库版本： 5.7.18 MySQL Community Server (GPL)
忘记密码，输入错误的密码时遇到下面错误信息：
[root@mytestlnx02 ~]# mysql -u root -p
Enter password:
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)
[root@mytestlnx02 ~]#
检查MySQL服务是否启动，如果启动，关闭MySQL服务
[root@mytestlnx02 ~]# ps -ef | grep -i mysql
root 22972 1 0 14:18 pts/0 00:00:00 /bin/sh /usr/bin/mysqld_safe --datadir=/var/lib/mysql --socket=/var/lib/mysql/mysql.sock --pid-file=/var/run/mysqld/mysqld.pid --basedir=/usr --user=mysql
mysql 23166 22972 0 14:18 pts/0 00:00:00 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib/mysql/plugin --user=mysql --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock
root 23237 21825 0 14:22 pts/0 00:00:00 grep -i mysql
[root@mytestlnx02 ~]# service mysqld stop
Stopping mysqld: [ OK ]
[root@mytestlnx02 ~]#
找到MySQL的my.cnf配置文件，在/etc/my.cnf (有些版本是/etc/mysql/my.cnf）在里面增加下面一段信息：
[mysqld]
skip-grant-tables
然后启动MySQL，进入MySQL后，修改root密码,操作过程中遇到ERROR 1054 (42S22): Unknown column 'password' in 'field list'，查了一下user表的表结构，发现原来MySQL 5.7下，user表已经没有Password字段。加密后的用户密码存储于authentication_string字段。具体操作过程如下所示：
[root@mytestlnx02 ~]# service mysqld start
Starting mysqld: [ OK ]
[root@mytestlnx02 ~]# mysql -u root
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.18 MySQL Community Server (GPL)
Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> use mysql;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> update user set password=PASSWORD('Kd8k&dfdl023')
-> where user='root';
ERROR 1054 (42S22): Unknown column 'password' in 'field list'
mysql> update mysql.user set authentication_string=password('Kd8k&dfdl023') where user='root';
Query OK, 1 row affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 1
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
mysql> exit
在my.cnf文件中，把刚才加入的那一行“skip-grant-tables”注释或删除掉。 然后重启MySQL服务